Text anzeigen (PDF) - Universität Duisburg-Essen
Text anzeigen (PDF) - Universität Duisburg-Essen
Text anzeigen (PDF) - Universität Duisburg-Essen
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
moment (µ B /EuO)<br />
4.1. Coherent growth: EuO on YSZ (100) 63<br />
(a)<br />
M(T) / M(T=5 K)<br />
1.0<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0.0<br />
0<br />
M in [100]<br />
M in [001]<br />
20<br />
40<br />
M<br />
M<br />
20 nm EuO/YSZ(001)<br />
magnetization, H=10 Oe<br />
60<br />
temperature (K)<br />
T C =70 K<br />
80<br />
(b)<br />
20 nm EuO/YSZ(001)<br />
hysteresis, T=5 K<br />
-400 -200 0 200 400<br />
field (Oe)<br />
H c =343 Oe<br />
H c =329 Oe<br />
6<br />
4<br />
2<br />
0<br />
-2<br />
-4<br />
-6<br />
Figure 4.4.: Magnetic properties of a 20 nm single-crystalline EuO (100) film with M measured inplane<br />
and out-of-plane. The measurement was conducted by B. Zijlstra.<br />
All EuO lattice parameters agree well with bulk EuO. In conclusion, we confirm a seamless<br />
coherent growth of EuO on conductive YSZ (100) by HR-TEM.<br />
A key property of EuO is its ferromagnetic behavior. In Fig. 4.4, the temperature dependent<br />
magnetization curves and the hysteresis curves are depicted for a 20 nm EuO thin film coherently<br />
grown on YSZ (100). Both in-plane and out-of-plane magnetization curves follow the<br />
shape of a Brillouin function. A large difference is observed in the magnetic switching behavior:<br />
the coercive field for in-plane magnetic switching along the [100] direction shows a low<br />
value of H c = 43 Oe which is indicative for a good crystallinity of EuO. For the out-of-plane direction,<br />
in contrast, the coercive field exhibits an eight times larger value of H c = 329 Oe, and<br />
the saturation magnetization cannot be reached. The reduced magnetization in out-of-plane<br />
direction can be explained by the significant fraction of interface and surface layers of EuO<br />
with reduced nearest neighbor coordination, as predicted by Schiller and Nolting (2001). <br />
Mainly three different anisotropy contributions determine the magnetic switching: crystalline<br />
anisotropy, shape anisotropy and pinning by defects. The crystalline anisotropy is<br />
weak in EuO, and the shape anisotropy is dominant. We distinguish between in-plane and<br />
out-of-plane anisotropy. A measure for the magnetic anisotropy is the anisotropy constant in<br />
first order K 1 , expressed as 54<br />
K 1 = −1/2H an σ sat . (4.1)<br />
Here, H an denotes the anisotropy field which is necessary to saturate the magnetic sample<br />
to the saturation moment σ sat in the magnetic easy direction, to which H an is parallel.<br />
For our single-crystalline EuO thin film, we determine the out-of-plane anisotropy as<br />
K1 ⊥ (cryst. film) = −0.851 × 105 erg/g. We compare this out-of-plane anisotropy with a polycrystalline<br />
EuO thin film (d = 100 nm) from literature, 150,151 for which K1 ⊥ (poly. film) =<br />
−9.3 × 10 5 erg/g was found. The large difference of magnetic anisotropy between polycrystalline<br />
and single-crystalline EuO thin films can be explained by magnetic pinning: while<br />
in polycrystalline EuO the pinning of magnetic moments due to crystalline defects is omnipresent,<br />
in single-crystalline EuO without defects, one would expect this pinning effect to<br />
vanish. Indeed, in single-crystalline EuO the shape anisotropy is smaller by a factor of ten<br />
than for the polycrystalline EuO film.<br />
For EuO thin film effects, please see Fig. 2.8 on p. 15.